1. Field of Invention
The present invention relates to fiber optics technology, and more particularly, to dynamic tilt gain equalizers.
2. Description of Related Art
In recent years, fiber optic communication systems have become increasingly popular for data transmission due to their high speed and high data capacity capabilities. Multiplexing the data transmitted via a fiber maximizes the transmittable data volume. Particularly, Wavelength Division Multiplexing (“WDM”) systems increase the transmission data rate through single-mode optical fiber by simultaneously propagating light from spectrally different but equally powered laser sources through the fiber.
Moreover, in WDM optical links, it is important to keep the signals of all the channels in a fiber at the same power level in order to avoid signal-to-noise ratio degradation due to the gain characteristics in optical amplifiers. This is difficult to accomplish because the non-flat gain profiles over the desired spectral ranges in optical amplifiers cause variations in power levels for different channels.
In a configuration of cascaded optical amplifiers in a WDM link, lower accumulated gain in certain wavelengths reduces signal-to-noise ratio, and this ratio limits the transmission distance. This problem may be resolved by installing fixed-gain filters in each amplifier to achieve a flattened gain. However, the gain profiles in the amplifiers vary in accordance to the number and power levels of the channels; and in a dynamically reconfigurable WDM network, the gain profiles of optical amplifiers will vary with network reconfiguration. Furthermore, even for simple point-to-point fixed add/drop WDM systems, there are design considerations relating to future addition of channels or reduction of WDM wavelength spacing. Thus, the gain profiles will vary as the number of channels varies.
If the gain of an optical amplifier is linearly dependent on the wavelength of the amplified signal, this dependence is known as the “gain tilt” of the amplifier. Therefore, when a WDM signal light is amplified by an optical amplifier (e.g. erbium-doped fiber amplifier, etc.), each of the signals of the individual channel may be amplified with a different gain.
The gain tilt effect occurs when the input power or channel numbers changes. FIG. 1 illustrated a positively sloped gain tilt denoted S1, a flat gain tilt denoted S3, and a negatively sloped gain tilt denoted S5. Typically, positive sloped gain tilt S1 occurs the most frequently, and for a WDM system, this gain tilt must be flattened. Therefore, with the fast-growing interest in dynamic reconfigurable WDM networks and scalability considerations, dynamically controlled optical gain equalizers become essential elements for the next generation optical networks.
In an effort to equalize the gain tilt, several methods have been developed for optical power equalizers. Some approaches separate the WDM channels and adjust each individually. This can be done in a first method by using a multiplexer/demultiplexer pair such as a phased array grating with an array of liquid crystal variable optic attenuators (“VOA”). The use of such a dynamic gain tilt equalizer (“DTGE”) can flatten the gain tilt, but such equalizers are complex and costly. FIG. 2 illustrates one method of flattening gain tilt by using this type of DTGE whereby the c-band is separated into four different windows. Subsequently, each window of channels goes through a corresponding WDM such that channels (“λ”) 3 to 9 go through WDM1, λ13 to λ19 go through WDM2, λ23 to λ29 go through WDM3, and λ33 to λ39 go through WDM4. After passing through its corresponding WDM, each window also goes through a corresponding VOA to adjust optical loss as shown in FIG. 2. Although the method shown in FIG. 2 roughly flattens a gain tilt, it has many disadvantages: 1) it incorporates too many components such as the WDM's and VOA's shown in FIG. 2; 2) the gaps between WDM's miss some of the channels; and 3) the gain tilt for channels in the same window is not eliminated.
An alternative method for dynamic tilt gain equalizer uses all-fiber, acousto-optic tunable filter (“AOTF”) technology. An all-fiber AOTF system works by creating wavelength selective losses as signals travel through an optical fiber. The wavelength selective losses are induced by imposing a tunable small-amplitude acoustic wave on a short length of optic fiber. Each AOTF creates a “notch” or rejection band in the optical spectrum, whereby the notch position and depth is independently adjustable with software. Each tilt gain equalizer contains eight AOTF's in series to produce the desired attenuation profile over the c- or l-band. However, this method is also complex and costly due to the use of many super-sound generators.